溫度對卵磷脂與無機鹽類於低極性溶劑中自組裝結構與流變行為的影響

Abstract

卵磷脂在非極性有機溶劑中可以自組裝形成反球狀微胞，文獻記載添加適量的膽鹽或是特定種類的無機鹽類，可使卵磷脂在低極性溶劑中形成反式蠕蟲狀微胞，這種長鏈狀的微胞類似高分子鏈會在溶液中糾纏，造成溶液黏度大幅增加，甚至形成黏彈體或凝膠。過去已有學者探討過溫度對於疏水作用力及氫鍵誘導形成的蠕蟲微胞流變行為及其微結構的影響，結果顯示由於疏水作用力比氫鍵更不受溫度的影響，在微結構上，氫鍵所形成的蠕蟲微胞長度會隨著溫度上升而有急遽的縮短，反觀疏水作用力的蠕蟲微胞長度只會有些微的減短；在流變行為上，氫鍵作用力之蠕蟲微胞的高原模數、零剪切黏度和鬆弛時間皆會隨溫度有明顯的下降，反之，疏水作用力誘導的蠕蟲微胞，隨溫度上升，唯獨黏度和鬆弛時間會減少，但高原模數幾乎維持不變。而離子靜電作用力的強度正好介於疏水作用力及氫鍵之間，故本研究主要欲探討溫度對於離子作用力所誘導之反式蟲狀微胞的影響，我們主要挑選三種不同離子強度的鹽類作為我們實驗的對象，分別是氯化鋰LiCl (一價陽離子)、碘化鋰LiI (一價陽離子+電子密度較低陰離子)以及氯化鈣CaCl2 (二價陽離子)，其離子作用力由高至低為CaCl2 > LiI >LiCl，此三種鹽類皆可誘導反球狀微胞形成反蠕蟲狀微胞，但由於作用力的強度不同，其誘導的能力也有差別，作用力越強越容易誘導反球狀微胞形成反蠕蟲狀微胞，在結果與討論的部分，我們額外也有將氫鍵所誘導形成的反式蠕蟲微胞加進來一起比較。我們使用流變儀和小角度X光散射技術去分析在不同溫度下流變性質與微結構的變化，並試圖從中找出不同種類的反式蠕蟲微胞與溫度的關係，流變行為方面，比較弱的離子作用力所形成的反式蠕蟲，(例如: LiI和LiCl)，其流變參數(如高原模數、鬆弛時間與零剪切黏度)會隨著溫度上升而有明顯的降低；而離子作用力比較強的反式蠕蟲(例如: CaCl2)，其鬆弛時間與零剪切黏度會隨溫度下降，但高原模數則維持不變；微結構的部分，三種鹽類形成的反式蠕蟲微胞，其長度皆會隨溫度上升而變短，離子作用力越弱的鹽類，其所形成的反式蠕蟲微胞的長度縮短的程度越顯著。 卵磷脂與鹽類的作用力是驅動形成反蠕蟲微胞的原因，因此為了釐清不同強度的離子作用力和溫度的關係，我們藉由傅立葉轉換紅外線光譜儀來研究卵磷脂與添加物之間的作用力對於溫度的變化，我們發現不管是哪種鹽類，主要都是與卵磷脂的磷酸根(PO4-)產生作用力，隨著溫度的上升，鹽類與卵磷脂間的作用力會因為溫度升高而有減弱的趨勢，導致某些特定官能基的吸收峰會產生位移。

Previous studies have shown that addition of bile salts or inorganic salts can transform lecithin organosols into viscoelastic fluids or even organogels in the low-polar solvents, where lecithin are induced to form wormlike micelles similar to long and entangled polymer chains. While the hydrophobic interaction is the driving force for the formation of micelles in water, the weak interactions, such as hydrogen binding and ionic interactions, are the driving forces for the amphiphilic molecules to self-assemble into reverse micelles in organic solvents. It has been reported that the effects of temperature on the rheological properties of hydrophobic interaction- and hydrogen bond-induced worm-like micelles in water and in low-polar solvent are different, which is due to the different responses of the hydrophobic interaction and hydrogen bond to temperature. For the worms driven by hydrophobic interaction, the plateau modulus Gp remains constant with temperature, and the relaxation time tR and zero-shear viscosity η0 drop exponentially. For the worms caused by hydrogen bonds, in addition to tR and η0, Gp also decreases exponentially with temperature. In this work, we studied the effects of temperature on the worms induced by ionic interactions which are stronger than hydrogen bonding. We focus on three inorganic metallic salts, including lithium chloride (LiCl), lithium iodide (LiI) and calcium chloride (CaCl2), all of which can transform lecithin organosols into organogels in low-polar solvents. The effectiveness of the salts to induce organogelation is positively correlated to the binding strength between salts and lecithin, in order of CaCl2 > LiI > LiCl. We found that the decay rates of Gp upon heating for the viscoelastic fluids induced by the three salts follow the same order CaCl2 > LiI > LiCl and all decay slower than that driven by hydrogen bonding, indicating that the dependence of the modulus on temperature can reflect the strength of the driving forces. The small angle X-ray scattering (SAXS) profiles support the effects of temperature on the rheological properties of these reverse worms by evidencing that the contour lengths of the LiCl- and LiI-induced reverse worms decrease dramatically with temperature, whereas the contour length of CaCl2-induced worms only decrease slightly within the same temperature ranges. We also used Fourier transform infrared spectroscopy (FTIR) to investigate the changes of the interactions with temperature and found that the interaction between lecithin and salts indeed vary upon heating.